Steel + Aluminium melding?

The same corrosion process will freeze helicoils as well. I suppose a solid stainless insert would work - it would be frozen in place, but unless the pressure was enough to crush the insert, the screw would still be free to turn.

Joe Gwinn

But, helicoils, or solid inserts for that matter, are normally considered permanent installations and are usually locked in place in some manner. If they were to "freeze" no one would ever know it :-)

Solid inserts are probably fine -- like the Keenserts which are locked in by some square pegs driven into the threads parallel to the axis.

However, Helicoils are a spiral of diamond shaped steel (stainless?) which compress a little as they are screwed in. If the corrosion above happened, it might squeeze the coils in too tight on the installed screws.

Note that the Helicoils are screwed in by torque applied to a cross-bar at the leading end of the screw, letting what follows wind inwards to reduce the diameter until it stops going in. The first screw into it likely causes the coil to expand slightly. The cross-bar is normally broken off (at a designed-in weak point) once it is installed.

However -- it is possible to remove the insert. An arrowhead shaped point on a tool bites into the last turns of the coil, and winds it out -- again shrinking the rest of it to make it turn more easily. (No bets what happens in the presence of the corrosion described above, however. :-)

Enjoy, DoN.

Helicoils don't seal off the aluminum underneath. You'd still get corossion -- between the helicoil and the aluminum, which will squeeze the helicoil into the bolt.

That was why I suggested isolation mounts. You can buy them with aluminum on one side and whatever you want on the other.

Drill & tap the holes, apply some Noalox to the aluminum stud side and screw it down. Between the noalox and the rubber of the mount it should seal it pretty well.

Now you have a rubber isolated stud to mount the panel racks to. Gives you a stronger mount, the rubber will allow the pieces to flex a bit easier and it would be electrically isolated as well.

Is this the neoprene rubber that over 20 years degrades to a gooey black paste? with the structural integrity of marmalade

You can spec the isolation material. The rubber I normally use is the same used for engine mounts. You could also use urethane if you wanted less flex.

And helicoils that are intended to be permanent have a serrated portion and you install the helicoil and then use an expander to force the serrated portion into the parent metal. A bit more difficult to remove then your arrowhead tool will handle.